Importnace of CRAH Delta T In chilled water loops

[ShmoJoe]

Got a 4k sq ft DC with three CRAH units cooling to a air cooled chilled water plant. Trying to explain the importance of not have supply/return air mixing within the DC so that CRAH units take in only return air (High Delta T vs LOW). I understand that this will maximize the individual CRAH unit capacity, but how does this affect efficiency at the chiller level? At the end of the day, the chilled water load will be the same no matter how efficient the CRAH units are, correct?

Thanks,

ShmoJoe

 

[imok2]

When you have a low delta T with a load-based strategy that measures system load or indirect
indications of load such as return water temperature. So what happens when delta-T falls below design levels
and flow and load are no longer in synch? The flow-based control system will always make sure
loads are met by starting additional chillers and pumps to keep the primary system flow larger than the secondary flow. But this means that chillers are not fully loaded when delta-T is below design. For example, assume the system was sized for a 10°F delta-T on both the primary and secondary sides. If the system were at 50% load but the actual delta-T was only 5°F, all the chillers and primary pumps in the plant would have to operate to keep the flow up. This wastes pump energy and chiller energy since the chillers would all be operating at 50% of capacity, less than the 65% to 85% range where efficiency is typically maximized for fixed-speed chillers
Some DC with CRAH units will use return air taken fron the ceiling area by extending the return ducts into the cealing so as not using the room air as return and this strategy helps with a better delta T

 

[marcoh]

You do need consider how you control the supply air fan speed to match rack air flow requirements.

If it is constant volume fan, the rack air flow will must somehow mix the excess supply air air to maintain air flows. Eg if SA = 1000L/s, rack air flow = 600L/s, the additional 400L's SA will mix with the warm rack discharge air.

The total chiller load will be close enough to being the same.

 

[ShmoJoe]

Thanks guys...

System is a pure constant flow (CRAH fan and chiller loop GPM of approx 320). As such, the only component of efficiency I can see is the compressor staging on the chiller. At current, with a 7 to 8 degree loop delta, the 1st set of compressors stay on constantly while the secondary bank kicks on for 15 - 20 minutes, then off for 10 15 minutes.